Saturday, December 8, 2018

astrophysics - Star Surface Temperature Vs. Mass


I am hoping that someone can clarify this for me.



With these equations. (Boltzmann Law) and radius of Radius of star how does surface temp scale with mass



$R _x \approx R_\bigodot (\frac{M_x}{M_\bigodot})^.5$


$L _x \approx L_\bigodot (\frac{M_x}{M_\bigodot})^3.5$


$\frac{L}{\pi R^2} = \sigma T^4$


$T_\bigodot = 5800 K$ = Surface Temp of Sun



What I have done is that I have substituted $R_x$ and $L_x$ in the 3rd equation and simplified it in terms of $T$ where my final expression is



$T = (\frac{L_\bigodot M_x^2.5}{\pi\sigma M_\bigodot^2.5 R_\bigodot})^0.25$




Am i done? What am I suppose to do next? Thanks



Answer



Notice that


$$\frac{L_{\bigodot}}{\pi R_{\bigodot}^2} = \sigma T_{\bigodot}^4$$


So that, dividing through the relation for an arbitrary star and that for the Sun gives:


$$\frac{L/L_{\bigodot}}{R^2/R_{\bigodot}^2} = T^4/T_{\bigodot}^4$$


Using the other relations


$$\frac{(M/M_{\bigodot})^{3.5}}{M/M_{\bigodot}} = (T/T_{\bigodot})^4$$


or



$$\left(\frac{M}{M_{\bigodot}} \right)^{2.5} = \left(\frac{T}{T_{\bigodot}}\right)^4$$


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